Field of the Invention:
The present invention relates generally to improved nematic liquid crystal elements and, more particularly, to such elements consisting of two parallel opposed plates, electrodes on the inner faces of each of the plates, and a thin nematic liquid crystal layer disposed between the plates.
Description of the Prior Art:
Recently, nematic liquid crystal display elements (hereinafter referred to merely as an "element") have been utilized as display elements and light modulation elements by causing the nematic liquid crystal (hereinafter referred to merely as "liquid crystal") to scatter light upon the application of an electrical field, thereby forming various patterns. These elements and display devices, and liquid crystal compounds used therein, are disclosed, for example, in U.S. Pat. Nos. 3,322,485 and 3,499,702. A fundamental structure of the element, a fundamental wiring for applying the field to the element, and types of electrode materials of the element are disclosed in these U.S. Patents. In the case where the element is incorporated in a practical device or equipment, the element is subject to various conditions. For example, the ease with which the element may be incorporated in the equipment depends on the space occupied by the element, the position the element is to be mounted, the positions of connecting wires, and methods used for connecting the wires. Moreover, the light rays are directed on the element to make patterns obvious when an electric field is applied to the liquid crystal layer, but it is influenced by the quantity of the light reflection and light transmittance of the electrode films. Furthermore, in the past the elements were driven by the application of an A.C. field to prolong the life of the element. The use of a D.C. field is advantageous with respect to the life of the associated circuits and also permits the use of a battery; however, the prior art has not solved the problem of prolonging the life of the element when D.C. is used.
When incorporating an element in practical equipment, the wiring between the element and equipment can be effectively carried out by mounting all of the electrode terminals on the inner face of one plate. In the case of the above mentioned U.S. patents, where the electrodes are crossed, the plates are shifted relative to each other and the electrode terminals are mounted on separate levels (namely, not being on the same level and parallel to each other) and wires extending in different directions connect the terminal to driving equipment; however, there are practical disadvantages of such a technique, as follows:
1. Because of the terminals being positioned on both plates at different levels and directions, the total package size for the element becomes large due to the shifted portions of the plate; namely, terminal areas are necessary for connections, and they occupy additional space, as compared with the case in which the terminals are positioned on the same plate.
2. When incorporating the element in equipment, the wiring from the driving equipment or circuit to terminals becomes complicated because of connections in two directions. The use of connectors requires two members and is therefore more expensive even when the terminal member is the same.
3. It is necessary to determine a contact position of the connectors and sockets twice and, further, mounting operations and connections increase. Moreover, even when electrode terminals are mounted on the same side on the inner face of each plate, connecting lead wires, lead frames or connectors to each of two opposing electrodes is extremely difficult because the space between two plates of the element is within the order of 5 to 50 .mu. in practice. Therefore, all of the electrode terminals should be mounted on one plate. To effect this purpose, the electrode terminal which is insulated from the other electrode on one plate is mounted on the edge of the said plate, and the electrode which is partially opposite to the said insulated electrode terminal is mounted on the other plate. Interposed between the electrode terminal on one plate and the electrode on the other plate is a metallic foil, such as aluminum foil, equal in thickness to the insulating spacer or several microns thicker than it or, in the alternative, a coating of electrically conductive paste over a portion of the insulating spacer is to provide position-transference of the electrode terminal (hereafter referred to merely as the "transfer connection").
However, the above connecting method has the following disadvantages and therefore is extremely low in the electrical reliability and often yields failures:
1. As the liquid crystal penetrates into minute apertures between the metallic foil and the electrode, insulation between them or increase in electrical resistance due to decrease of contact area of the metallic foil with the electrode occurs. Therefore, the desired results cannot be attained.
2. The thickness of a liquid crystal layer cannot be arbitrarily selected because a thickness of the metallic foil must be equal to that of the spacer or slightly thicker than the latter.
3. Where the transfer area, namely the area of an electrode terminal to which a transfer connection is to be made, is small, the interposing of the foil is quite difficult.
4. As the interposed foil is not reliably fixed and may shift in the liquid crystal, it is apt to move from a position at which the transfer connection of an electrode terminal is effected and to contact with other electrodes, thereby causing a short or cross charging effect or not effectively performing the connection.
5. There are some difficulties in manufacture in that the foil must be interposed between two electrodes before filling the liquid crystal.
6. Manufacture of the element needs a skilled worker.
Also, in the case where an electrically conductive paste is used for the metallic foil, there are the following disadvantages:
1. The conductive paste is apt to dissolve in the liquid crystal because of directly coming into contact therewith and disperse therein thereby causing a short.
2. The dissolution of the conductive paste affects the property of the liquid crystal.
3. There are some difficulties in manufacture in that the conductive paste must be coated with the uniform thickness on the given small area.
4. In manufacturing the element, it is necessary to pile electrode patterns mounted on the plates opposite to each other simultaneously with coating the paste which requires a great deal of skill.
On the other hand, as the liquid crystal itself does not emit light by the application of an electrical field or current, there is need to utilize a light source, such as daylight, room light or a spot light when incorporating the element to the equipment. In such an element, a plate and electrode on the front side thereof must necessarily be transparent, while a plate and electrode on the back side thereof may be transparent or reflective. The term "the front" used hereinafter in relation to a pair of plates of the element and an electrode mounted on the inner face of each of the plates is referred to as designating the plate mounted electrode directed to an observer side and the term "the back" as the plate mounted electrode positioned opposite to the observer side.
Transparent electrodes are, for example, obtained by vacuum depositing, for instance, tin oxide, indium oxide or tantalum on a transparent plate such as glass plate, while reflective electrodes are, for example, obtained by vacuum depositing, plate or printing a metal such as chromium, nickel, copper, lead, silver, gold aluminum, titanium or an alloy such as Inconel or a glass plate, ceramic plate or plastic plate. When the incident light is scattered by the liquid crystal layer under the application of the field, a portion of the light passes through the light scattering area, but in the case of using a reflective electrode as the back, this portion of the light is reflected through the scattering area, so that the contrast increases.
On the other hand, in the case of using a transparent electrode mounted on a transparent plate as the back, the light scattering by the liquid crystal is inferior to that in the reflective electrode in contrast and it is difficult to observe. Therefore, increase of contrast is effected by providing a source of light to the rear on the inside of an element-incorporated device and emitting the light at the proper angle.
Thus, in order to have the observer distinguish patterns, the light must be directed onto the element from the observer side when using the reflective electrode as the back, while in the case of using the transparent electrode as the back, the light must be emitted from the rear (namely opposite to the observer side). However, in the case of an element wherein the back is of a transparent electrode, a source of light is provided to the rear on the inside of the device as mentioned above and there is always need to turn on the light during operation of the element. Accordingly, there are economic disadvantages in that the source of light needs a great deal of energy and the transparent electrode is relatively expensive as compared with the reflective electrode.
Next, in the case of an element wherein the back is of a reflective electrode, if it is attempted to obtain an excellent contrast with daylight or room light only, the element must be observed in bright surroundings, if possible. But, where the surrounding is dark, it is impossible to increase the contrast. Accordingly, an illuminator is provided to obtain the required contrast to the front of the element. However, it is necessary to provide a space for receiving the illuminator to the front in the inside of the device, namely to the observer side of the element and therefore it is economically disadvantageous in that the size of the device itself becomes too large. Moreover, because of the need of housing for receiving the illuminator, the observation angle is limited, and the external light rays are obstructed by it.
When incorporating a liquid crystal element filled with a thin liquid crystal layer between a pair of electrodes into a device to be used in practice, practical problems of the life of the element become most important. The liquid crystal causes light scattering under the application of either direct current or alternating current voltages above a threshold value.
On the other hand, from the aspects of a circuit for driving the element or costs thereof the application of D.C. or a D.C. pulse field is quite desirable. The advantages are that the element can be driven with low voltages and also that the low consumption of power are compatible with portable instruments utilizing a battery. However, when the element is driven under the application of D.C. or a D.C. pulse field, particularly in the initial step of the application, foams emerge in the element and the liquid crystal itself turns yellow thereby becoming impossible to use. In the case that an electrode of the generally used metals, such as aluminum, chronium, copper, gold or silver is utilized as a positive electrode under the application of D.C. field, the resulting anodic oxidation and effluence of the metals have a bad influence upon the liquid crystal, and also the electrode is often peeled off. The electrode of the above metals can be used as a negative electrode only in order to prevent these disadvantages although it is of no use to prevent emergence of foams and yellowing of the liquid crystal. Also, even when the negative electrode is made from metal oxides such as tin oxide or indium oxide, the foaming or yellowing occurs. These drawbacks, namely the emergence of foams and the yellowing of the liquid crystal have been considered due to impurities present in the liquid crystal or properties of the liquid crystal itself, but this has not been made clear yet. Introductions of a novel additive to the liquid crystal or the synthesis and improvement of the liquid crystal have been studies, but the useful ones have not as yet been found.